Put the egg whites and sugar in a saucepan. Heat up and stir constantly until the mixture is warm to the touch.

Transfer to a mixing bowl and whisk until stiff peak.

Still mixing, add in the butter a couple at a time.

Once everything is added in, continue mixing. The mixture won't look like buttercream at this stage, but continue mixing. The mixture would even turn into like a curddled cream, don't worry it's normal. Continue mixing until it will eventually become buttercream.

You can also add flavor to this buttercream by adding jams, flavor extracts of your preference or 90 g of melted chocolate.